Many electrical assemblies call for one component to be attached to another component with a minimum of electrical resistance between said components. Mechanical fastening may be necessary for exceptionally heavy parts, but it is unsuitable for many other uses because it complicates design, adds considerable weight and is costly. Conformal coatings are suited only for bonding very-low-mass components which are not subject to great mechanical forces. Consequently, for many applications adhesive bonding has proved to be the most feasible and economical method for securing one electrical component to another. For example, magnetic read/write heads which are extensively used in computers and other applications in which data is written onto and read from a magnetic medium such as a magnetic tape or disk are bonded onto suspension assembly arms which are computer controlled so that the head may be rapidly moved from position to position to write or read information from the portion of the medium specified by the computer. Hence, the adhesive bonds between the head and arm also must be strong. The strength requirement follows from the fact that the magnetic heads must be moved as rapidly as possible over the associated magnetic medium in order to minimize the time required for locating the correct track that is to be read or written. This rapid movement often produces acceleration and deceleration forces on the head on the order of 40 gs. This mounting job also calls for minimal electrical resistance between the read/write head and the suspension assembly arm. This requirement follows from the fact that one does not want an electrical charge such as those created by static electricity to build upon the head. Such a charge build-up on the head may cause data collection and transfer malfunctions. Hence minimal electrical resistance between the head and the suspension arm serves to prevent a build-up of a net charge on the head.
The techniques by which read/write heads are bonded to their respective suspension assembly arms are, to a large degree, dictated by the levels of precision required by the magnetic disk environment. For example, magnetic disks must be manufactured with an extremely high degree of flatness because the head is called upon to "float" on a very thin film of air just above the disks. Therefore the head is secured to a resilient suspension assembly arm in order to obtain the mobility which permits the head to float on the air film and to provide a resilient load which exerts an accurately defined pressure on the head.
Moreover, the current trend in the industry is towards higher and higher recording densities and an ever increasing numbers of tracks on the associated disks. This has led to increased miniaturization of all parts of the recording apparatus including the heads and the suspension arms to which the heads are attached. As a result, thin film heads are currently used that have dimensional tolerances on the order of several millionths of an inch. This requires that the heads be accurately positioned on the associated suspension arm so that the head may operate parallel to the plane of assembly arm as well as the plane of the disk with which it is to exchange data. In practice, the head must be positioned parallel to the plane of the disk within several thousandths of an inch in each dimension. The head must also have a minimal rotational error with respect to the magnetic orientation of the data to be read. These are all important considerations because the head is separated from the moving disk by no more than a few millionths of an inch. All this goes to say that the positioning of a read/write head on a suspension arm is extremely critical with respect to the tolerances which must be achieved if the optimum operation of the head is to be achieved. Such close tolerances and accurate positioning requires that heads be positioned with extreme precision on suspension assembly arms.
The typical prior art procedure used to affix a head to a suspension arm is to first position the suspension arm in a first tool holder. The head is placed in a second tool holder and the two tool holders are then moved in a controlled manner so that the end of the suspension arm and the head are brought together with the required orientation. The head is then permanently affixed to the end of the suspension arm. A bonding or "gluing" the head to the suspension arm has evolved as the most practical method of fabrication. One of the most common procedures is to use a dispensing device such as a syringe to lay a strip of adhesive along the junction defined by the end of a vertical face of the head and the horizontal surface defined by the top surface of the suspension arm. That is to say the strip takes the form of a fillet in the angle formed by the junction of the side of the head and the top of the suspension arm. Typically the bonding adhesive is laid along at least two of the junctions defined by say a rectangular shaped head and the flat surface of the suspension arm to which the head is mounted.
The adhesives employed for this purpose are normally epoxy resins having a high solid content ("high-solid-content adhesives"). High-solid-content adhesives of this type usually have a liquid and/or a solid component which is an epoxy resin of from at least 90 weight percent to as high as about 98 weight percent of the adhesive material. A solid curing agent can be employed in either a liquid and/or in a solid resin. Such resins are also often referred to in the literature as "100 percent solid resins". That is to say these epoxy resins are designed to cure without releasing very much moisture or other by-products. In some formulations no solvents necessary. Such 100 percent solids adhesives are employed in head/suspension arm bonding operations in order to reduce the possibility of entrapped gases, voids, and porosity in the bond line which would significantly lower bond strengths. Moreover, solvents and entrapped moisture are known to cause serious corrosion problems in metal bonding. This is the reason it is possible to bond successfully to metal with epoxies using only contact pressures, because it is unnecessary to provide for water removal (as in phenolics) and solvent removal (as in elastomer emulsions). The 100 percent solids content of epoxy also makes it convenient for rapid assembly-line bonding to nonporous surfaces such as glass and metals. Common examples of such high-solid-content adhesives include epoxy (anhydride cure), epoxy (amine cure), epoxy phenolic, epoxy polyamide and epoxy nylon formulations. Adhesives of this kind have a thick glue-like consistency and usually take several hours to completely dry at room temperature. Hence, most bonding operations of this type are speeded up by the use of heat applied to the adhesive fillet. At temperatures greater than about 400.degree. F. these high-solid-content adhesives usually take from at least one to about three minutes to permanently set.
It should be specifically noted that these high-solid-content adhesives are not normally placed at the interface of the bottom of the head and the top surface of the suspension arm. This prohibition follows from the fact that such high-solid-content adhesives do not spread evenly and/or remain in too thick a layer and hence tend to create poor electrical contact between the bottom surface of the head and the suspension arm. Thus, as was noted above, read/write heads are typically bonded to the top of their associated suspension arms by strips of adhesive laid as fillets at the junction of the two electrical components.
This fillet bonding or gluing procedure has certain inherent problems, especially when it is applied to head/suspension arm affixing operations. Heads and arms are positioned by aligning edges and using tooling holes. These are hand positioned by an operator. As previously noted, the accuracy by which the head is positioned on the arm is dependent upon a number of variables. These include, the dimensional accuracy of the head, the accuracy of the tool that holds the arm, the accuracy of the tool that holds the head, the accuracy of the apparatus that moves the head over the arm, and finally the accuracy of the apparatus that lowers the head onto the arm. Thus, in the head positioning operation, it is necessary that the geometric center of the head be positioned with precision over a predetermined point on the assembly arm with an error of no more than plus or minus one mil in any direction and with an error of only several seconds of a degree with respect to the rotational position of the head with respect to the plane of the top surface of the assembly arm. Although the design objectives are plus or minus one mil, in a typical production line, there can be an error of five mils due tolerances of the head and the apparatus used to position the head on the suspension arm. An error of in excess of two mils can cause operational problems that require head-arm assemblies having errors in excess of this amount to be discarded. Needless to say, the equipment, such as the hereinafter described optical alignment equipment used to position and hold a read/write head upon a suspension arm while the high-solid-content adhesive is cured is extremely expensive. Hence, those skilled in this art will appreciate that any decrease in the residence time of those electrical components which are undergoing bonding in expensive equipment of this type implies great economic advantage. The adhesive formulations of this patent disclosure, as well as the methods of employing them, are aimed at improving the bonding operations associated with mounting read/write heads to suspension arms, but those skilled in this art will appreciate that the compositions and methods hereinafter described can be successfully employed in a multitude of electrical assembly situations. They are especially useful where there is a need for one electrical component to be precisely bonded to another electrical component with minimal electrical resistance between said components and with minimal amounts of residence time in the expensive equipment normally needed to achieve high levels of precision in mounting one electrical component to another.